The present invention relates to a silicon photomultiplier having an improved performance through the structure improvement.
Typical silicon photomultipliers basically have a planar diode structure and is generally composed of doped layers of p+-p-n+ or n+-n-p+. As the reverse voltage applied to such a diode structure is increased, the depletion layer between p+ and n+ is expanded, and when the voltage becomes higher than the breakdown voltage, an electrical breakdown occurs.
The avalanche mode is an operation mode using the phenomenon that, when the above device operates as a photo-sensor with an applied voltage a little bit lower than the breakdown voltage, input photons are changed into pairs of charge carriers inside the semiconductor by photoexcitation, and then the charge carriers are multiplied by a gain of dozens to hundreds through the impact ionization occurring in consequence of carrier acceleration by the internal electrostatic field.
Meanwhile, it is possible that the breakdown actually does not occur immediately after a reverse bias higher than the breakdown voltage is applied, if there is no carrier excitation by external stimuli or thermal activation. When an avalanche photodiode is operated at a bias that exceeds the breakdown voltage, it is said to be in the Geiger mode. When a pair of charge carriers is generated by photoexcitation in the photodiode operating in Geiger mode, the carrier number may increases geometrically by successive impact ionization caused by the electrostatic field, and the stored charge in the diode structure may flow instantly. Through this phenomenon, very high current-to-photon gain can be obtained. Consequently, detection of a single photon incidence becomes possible.
The advantage of this operating method is that such highly sensitive detection of a single photon level signal is possible even with a relatively simple device structure. For this operating method, the device is composed of multiple cells of the diode connected in parallel, and the number of photons entered the device is determined by the strength of current pulse generated after the photon incidence.
Accordingly, the dynamic range of light intensity measurement for such a Geiger mode photomultiplier depends on the area of a single cell to the entire area of the device, and the number density of cells. Therefore, a cell pattern of high number density is required to increase the above dynamic range.
It should be noted that the cell diodes in the device need to be recovered from the breakdown in a short time. When an electrical breakdown occurs in a cell, the cell cannot operate for a while and if a large current continuously flows through the cell after the breakdown, the device may be damaged. To prevent this, a method of passive quenching that a quench resistor is connected in series to the diode structure of each cell is generally used. The quench resistor plays a role of quenching the breakdown and recovering the cell into a stand-by state, by inducing Ohmic voltage drop when instant current by the electrical breakdown flows, thereby momentarily reducing the voltage applied to the cell diode below the breakdown voltage.
FIG. 1 is a schematic plan view illustrating a conventional silicon photomultiplier device, and FIG. 2 is a cross-sectional view of the device in FIG. 1, taken along the line a-a′.
Referring to FIG. 1, it shows cell light-receiving parts 101, quench resistors 102, metal electrodes 103, and electrode contact parts 104.
A cell light-receiving part 101 of a silicon photomultiplier device comprises a planar diode structure, in which an electrical breakdown triggered by photoexcitation of charge carriers can occur.
The metal electrodes 103 connect upper bonding pads of a device to the quench resistors 102 and connect each quench resistor 102 with a diode part in the corresponding cell.
The electrode contact parts 104 represent contact areas formed to provide the metal electrodes 103 with electrical connection paths that pass through the insulating layer to other parts.
An entire device has a structure that plural cells are connected in parallel, as shown in FIG. 1.
Referring to FIG. 2, it shows a quench resistor 102, metal electrodes 103, an upper doped layer 105, an intermediate layer 106, a buried layer 107, an insulating layer 108, and a guard ring 109.
Here, the upper doped layer 105, the intermediate layer 106, and the buried layer 107 show the inside of the diode structure of the cell light-receiving parts 101 by dividing the structure according to the doping degree and the role.
The upper doped layer 105 is a part doped with n+ and the impact ionization occurs most strongly right under this area.
The intermediate layer 106, which is a part lightly doped with p, corresponds to a part of the original substrate or is a part grown by silicon epitaxy after formation of the buried layer 107.
The buried layer 107 is a part doped with p+ that operates at the same potential as that of the electrode under the substrate, and is formed by the high energy ion implantation, by silicon epitaxial growth with a high content of dopant, or by other diffusion methods.
The insulating layer 108 is generally formed of an insulator such as silicon dioxide or silicon nitride to protect the surface of the substrate and separate the substrate from the quench resistors 102 and the metal electrodes 103.
The guard ring 109 plays a role in preventing the premature breakdown by reducing the electric field strength near the edge portions of the upper doped layer 105.
A point to note when measuring the number of entered photons using such Geiger mode silicon photomultiplier is that the spreading of the effect of photon incidence signal generated from a cell to adjacent cells should be suppressed properly. When an electrical breakdown occurs in a cell by the avalanche phenomenon, a plurality of photons may be generated from the accompanying impact ionization, etc. It is known that out of photons generated from 100,000 impact ionization event, about 3 of them have higher energy than the silicon gap energy. If the high energy photons reach the adjacent cells, additional breakdowns can occur in the adjacent cells. If the probability of such an additional breakdown were close to 1, the breakdown would successively spread to the adjacent cells, making the multicell design of the device pointless.
Accordingly, to suppress such an optical crosstalk, a method of spacing cells diodes out from each other is generally used in the existing arts, and additionally, a method using a trench isolation structure has also been proposed.
A method for suppressing the optical crosstalk by widening the space between cell diodes as mentioned above may be the most natural solution. However, such a design is not adequate for silicon photomultipliers with small cell sizes. For a silicon photomultiplier with a relatively large cell number density and a small cell size, the effective light-sensing area in comparison to the entire device area is considerably reduced because of the area occupied by the above mentioned quench resistors, metal electrodes, and the space between the cell diodes. Such reduction of the effective fill factor deteriorates the photon detection efficiency of the device.
Accordingly, a trench isolation structure has ever been proposed to effectively block photons even when the space between the cell diodes is narrow. In this structure, photons leaking out to the adjacent cells are blocked by optical isolation trenches placed between the light-receiving cell diodes. The optical isolation is based on the reflection of light at interfaces between materials of different refractive indices. The trench structure is formed by etching the silicon substrate and covering the etched surface with a layer of insulating materials such as silicon dioxide or silicon nitride.
The technologies described above denote the background arts in the technical field to which the present invention belongs, and do not denote the conventional arts.